std::numeric_limits<T>::quiet_NaN

 

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static T quiet_NaN() throw();		(until C++11)
static constexpr T quiet_NaN() noexcept;		(since C++11)

See More

Returns the special value "quiet not-a-number", as represented by the floating-point type T. Only meaningful if std::numeric_limits<T>::has_quiet_NaN == true. In IEEE 754, the most common binary representation of floating-point numbers, any value with all bits of the exponent set and at least one bit of the fraction set represents a NaN. It is implementation-defined which values of the fraction represent quiet or signaling NaNs, and whether the sign bit is meaningful.

Return value

T	std::numeric_limits<T>::quiet_NaN()
/* non-specialized */	T()
bool	false
char	0
signed char	0
unsigned char	0
wchar_t	0
char8_t (since C++20)	0
char16_t (since C++11)	0
char32_t (since C++11)	0
short	0
unsigned short	0
int	0
unsigned int	0
long	0
unsigned long	0
long long (since C++11)	0
unsigned long long (since C++11)	0
float	implementation-defined (may be NAN)
double	implementation-defined
long double	implementation-defined

Notes

A NaN never compares equal to itself. Copying a NaN may not preserve its bit representation.

Example

Several ways to generate a NaN (the output string is compiler-specific):

Run this code

#include <iostream>
#include <limits>
#include <cmath>

int main()
{
    std::cout << std::numeric_limits<double>::quiet_NaN()     << ' ' // nan
              << std::numeric_limits<double>::signaling_NaN() << ' ' // nan
              << std::acos(2)    << ' '   // nan
              << std::tgamma(-1) << ' '   // nan
              << std::log(-1)    << ' '   // nan
              << std::sqrt(-1)   << ' '   // -nan
              << 0 / 0.0         << '\n'; // -nan
    
    std::cout << "NaN == NaN? " << std::boolalpha
              << ( std::numeric_limits<double>::quiet_NaN() ==
                   std::numeric_limits<double>::quiet_NaN() ) << '\n';
}

Possible output:

nan nan nan nan nan -nan -nan
NaN == NaN? false

See also

has_quiet_NaN [static]	identifies floating-point types that can represent the special value "quiet not-a-number" (NaN) (public static member constant)
signaling_NaN [static]	returns a signaling NaN value of the given floating-point type (public static member function)
nannanfnanl (C++11)(C++11)(C++11)	not-a-number (NaN) (function)
isnan (C++11)	checks if the given number is NaN (function)